Problem: The scale on a map is 8cm : 3km. If two cities are 80cm apart on the map, what is the actual distance in km between the two cities?
Explanation: The scale means that every 8cm on the map corresponds to an actual distance of 3km. A distance of 80cm on the map is the same as 10 $\cdot$ 8cm. The actual distance between the two cities is 10 $\cdot$ 3km, or 30km.